Fluids (e.g., oil, water, gas) trapped in geologic formations are often recovered via a well, or borehole, drilled into the formation. A drilling operation generally utilizes a drill bit attached, as part of a bottom-hole assembly (BHA), to a drill pipe suspended from a surface facility. Drilling mud may be circulated through the drill pipe, drill bit, and an annulus formed between the pipe and borehole wall to cool the drill bit and carry drill cuttings back up to the surface.
During drilling, it is often desirable to monitor the properties of the borehole and surrounding formation and fluids. For this purpose, well logging tools may be integrated into the BHA, acquiring data in real time (or near real time) at increasing borehole depths as the drill bit advances; this technique is known in the industry as “logging while drilling” (LWD) or “measuring while drilling” (MWD). Different tools may be used for different types of measurements: for example, density and neutron tools may provide information about the porosity of the formation (allowing inferences about the probability of finding oil vs. gas); gamma-ray tools may help distinguish between different types of rock (e.g., sandstone and limestone); resistivity tools may use electrical resistivity measurements to determine whether water or hydrocarbons are present; sonic tools may measure the speed of sound in the rock, which is useful in planning fracking operations; and caliper tools may determine the size and shape of the borehole. Hole shape information may be used, for example, by drilling engineers to compute the volume of cement required to complete the well, by reservoir engineers and geologists to understand downhole stresses and the orientation of break-outs and fractures, and by petrophysicists to correct formation evaluation measurements performed with other tools (e.g., resistivity and neutron-porosity tools) for the effect of standoff (i.e., the distance of the tool from the borehole wall).
The data acquired downhole by the LWD tools may be transmitted in (near) real time up to the surface for processing and evaluation. From borehole depths beyond the practicable reach of wired communication, this can be achieved, e.g., via mud pulse telemetry, a technique that involves imparting pressure variations that encode the data in binary form onto the drilling mud. This mechanical form of transmission is subject to significant bandwidth limitations; in many implementations, data transmission rates are in the range from only three to ten bits per second. Accordingly, different logging tools may compete for bandwidth, and the information update rate provided by a given tool may be throttled to allow other data to be transmitted between successive updates.